A. Field of Invention
This invention relates to impact printing machines and in particular to a novel and improved ballistic print hammer assembly which is characterized by high velocity, short dwell time, low effective mass of the hammer head and low cost and ease of manufacture.
In impact printers the print hammer is actuated to cause an impact between a type carrier and a printing medium so as to result in a selected character being printed on the printing medium. In some printers the type carrier is stationary at the instant of impact and in others the type carrier is moving (on-the-fly) at the instant of impact. Because of its high velocity and short dwell time characteristics, the print hammer assembly of the present invention is especially useful in on-the-fly printers in which the type carrier is moving at such high speeds that short dwell times are necessary to avoid smear and/or tearing of the print medium. However, the simplicity of manufacture and low cost characteristics of the print assembly of this invention also make it attractive for lower speed on-the-fly printers and even in those impact printers in which the type carrier is stationary at the instant of impact.
In the context as used herein, the term "ballistic print hammer" means that at some point in the hammer's path of travel between its rest position and its printing position, the hammer is in a free-flight condition. This is generally achieved, for example, by initially applying an accelerating force to the hammer to move it from an initial rest position toward a printing position. However, before the hammer reaches the printing position the accelerating force is removed. Due to its inertia, the hammer then continues to move toward the printing position and to impact the printing medium with the type carrier.
B. Prior Art
One of the problems associated with the use of ballistic print hammers is that different amounts of kinetic energy of the hammer head are required to print single and multi-part forms. That is, more energy is required to print the multi-part form than to print the single part form. One prior art attempt to solve this problem involved the placement of damping pads at a forward stop location so as to absorb kinetic energy. A disadvantage of this technique is that it requires critical initial adjustments and also frequent field adjustments of the distance between the rest position of the hammer and the damping pads. Another prior art attempt to solve this problem involved varying the amount of energy applied to the actuator of the hammer. This generally involves applying a relatively high electric current for the multi-part form situation and a relatively low current for the single part form situation. The problem with this approach is that it changes the acceleration time of the hammer which in turn requires an elaborate and costly machine timing mechanism which can accommodate all of the acceleration conditions for all values of current employed in the approach.